**6. Reactor types**

102 Biogas

also showed synergistic effects during anaerobic treatment process. For instance, the presence of chromium in the sludge results in higher toxicity of copper (Colussi et al., 2009). Altas in 2009 showed that low concentrations of metals in the anaerobic reactor can be extremely toxic. Meanwhile, Cantrell and coworkers (2008) indicated that high concentrations of soluble metals have come to completely stop the production of biogas in an anaerobic system. To combat metal toxicity in the anaerobic degradation process, they

**Substance Total Concentration (mg/L) Soluble Concentration** 

Cu 200 0.5 Cr (VI) 50 – 70 3 Cr (III) 200 – 260 - Ni 180 - 420 2 Zn 30 1 Table 3. Concentrations of inorganic compounds inhibitory of anaerobic process (Yue *et al.*,

**5. Proprieties and volumetric compensation of the gases from anaerobic** 

The physical and chemical proprieties of biogas, affect the choice of technology used for clean-up and combustion; therefore, knowledge of these proprieties is useful for optimization biogas utilization. Since biogas contains primarily methane and carbon dioxide, this section is focused on their respective physical characteristics (Table 4). Because others components (nitrogen, hydrogen sulfide, traces organics), are present in relatively small quantities are not considered in the table. The magnitude of CH4 and CO2 varies greatly and depends on the composition of the organic material digested in the wastewater.

**Proprieties Methane (CH4) Carbon dioxide (CO2)a** 

0.0581 by mass

a: pure gas given at 77 ºF and 101.3 kPa; b: sublimes; c: Air at 101.3 kPa and 15.54 °C

Molecular weight (g/mol) 16.04 44.01 Specific gravity, (air =1c) 0.554 1.52 Boiling point, (14.7 psia) 126.0 °C 43.0 °C b Freezing point, (14.7 psia) -182.4 °C - 56.55 °C Specific volume (m3/kg) 1.51 7.03x10-3 Critical temperature (ºC) 46.62 31.08 Critical pressure (kPa) 4,640.1 7391.15 Heat capacity (kJ/kg K) 2.26 0.858

Ratio Cp/Cv 1012 Heat of combustion (kJ/m3) 377 Limit of inflammability 5-15% by volume Stoichiometry in air c 0.0947 by volume

Table 4. Physical constants of methane and carbon dioxide

**(mg/L)** 

can be precipitated as sulphate salts and carbonate salts, except iron and chromium.

2007).

**process** 

Many reactor configurations are used for the anaerobic treatment of agro-industrial wastewaters. Among them, the most common types are discussed and illustrated in Fig. 2.

Fig. 2. Most commonly used anaerobic reactors types: (A) Completely mixed anaerobic digester, (B) UASB reactor, (C) AFB reactor, (D) Upflow AF reactor (Ersahin et al., 2011)

#### **6.1 Completely stirred anaerobic digester**

The completely stirred anaerobic digester (CSTR) is the basic anaerobic treatment system with an equal hydraulic retention time (HRT) and solids retention time (SRT) in the range of 15-40 days in order to provide sufficient retention time for both operation and process stability. Completely mixed anaerobic digesters without recycle are more suitable for wastes with high solids concentrations (Tchobanoglous et al., 2003). A disadvantage of this system

Biogas Production from Anaerobic Treatment of Agro-Industrial Wastewater 105

fact clogging by biosolids, influent suspended solids, and precipitated minerals is the main problem for this system. Applications of both upflow and downflow packed bed processes can be observed. Prevention of methanogens found at the lower levels of the reactor from the toxicity of hydrogen sulfide by stripping sulfide in the upper part of the column and solids removal from the top by gas recirculation can easily be achieved in downflow systems in comparison to upflow systems. However, there is a higher risk of losing biosolids to the effluent in the downflow systems. Design OLR is often in the range of 8-16 kg COD/m3-d which is more than tenfold higher than the design loading rates for aerobic

Methanogenic anaerobic digestion is a classical anaerobic bioconversion process that has been practiced for over a century and used in full-scale facilities worldwide. This is a complicated process that involves a mixture of population of microorganisms and several gasses and liquid products, thus strict process control and product purification are required. Biogas production have been demonstrated in numerous studies with great success like can

> OLR (kg COD/m3-d)

distillery UASB -- 16.5-44.0 80 16.5 Shin et al.,

and dairy CSTR 2.0 5 90 -- Ince

leachate AFB 4.7-16 2.41-7.98 >90 --- Lin (1990)

Tequila is a Mexican regional alcoholic beverage obtained from the fermentation of sugars from the cooked stems of blue agave (Agave tequilana Weber var. azul). Its production and

and dairy CSTR 4-7 -- -- 0.55

Table 6. Typical performance of anaerobic reactor used for wastewater treatment

**8. Biogas production from agro-industrial wastewaters** 

AFB 5.6-32 4.65-20 85 0.168 Yeoh

reactor -- 10 98 -- Malaspina

reactor -- 0.97-2.82 91-97 0.28-0.35 Strydom et

COD removal (%)

CH4 Yield

(m3/kg COD) Reference

(1992)

(1997)

et al (1996)

al (1997)

(1998)

Yilmazer y Yeningüm (1999)

**7. Bioenergy production from different kinds of wastewaters** 

HRT (days)

processes (Rittmann and McCarty, 2001).

see in the Table 6 (Gavrilescu, 2005).

Type

Hybrid

Hybrid

Wastewater Reactor

Brewage

Canemolasses stillage

Cheese whey and dairy

Cheese whey and dairy

Cheese whey

Cheese whey

Landfill

(Gavrilescu, 2005)

**8.1 Case vinasses of tequila** 

is that a high volumetric loading rate is only obtained with quite concentrated waste streams with a biodegradable COD content between 8,000 and 50,000 mg/L. However, many waste streams are much dilute (Rittmann and McCarty, 2001). Thus, COD loading per unit volume may be very low with the detention times of this system which eliminates the cost advantage of anaerobic treatment technology. Typical the OLR for this digester is between 1-5 kg COD/m3-d (Tchobanoglous et al., 2003).
